1. Field of the Invention
This patent specification relates to positioning and/or orienting of downhole devices and, more particularly, to techniques for providing active positioning of downhole devices such as sensors using spherical motors such as spherical ultra-sonic motors.
2. Background of the Invention
In the field of borehole deployed devices, many components such as sensors significantly benefit or require downhole orientation and positioning. For example a downhole tiltmeter typically has at least two sensors mounted orthogonal to one another. Each of the sensors is typically individually adjusted to level or zero the sensors. For some known designs in downhole oilfield application the tiltmeter must be leveled once they have reached target depth. The tiltmeter sensors, thus have to be adjusted by certain amounts depending on the deviation of the well.
According to one known approach, the sensors are leveled by manipulating two cylindrical sleeves. See, U.S. Pat. No. 6,370,784. However, the two orthogonal sensors need to be installed at slightly different depths because of the size of the orienting mechanism. Additionally, the size of the sleeves requires that the tool housing is relatively long.
Other examples of known tiltmeter leveling techniques are described in U.S. Pat. No. 4,397,098 (Method and Apparatus for zeroing a Tiltmeter Mechanism) and U.S. Pat. No. 5,063,680 (Self-Erecting Tiltmeter).
Some known downhole inclinometers use a rigidly mounted 3-axis accelerometer to determine the gravity vector based on the 3-axis sensor output. In these designs it is essential to maintain scale factor consistency among 3 axes. Since the accelerometer scale factor changes due to temperature, compensation is required based on a temperature measurement and the pre-memorized calibration data acquired at surface for each individual axis of accelerometer. An optimistic assumption is often made that no change of the compensation coefficient is needed during the time between calibrations.
Another application where downhole orientation and positioning is used is with downhole gyroscopes. A downhole gyro can be used for north-seeking based on Earth spin detection. Some designed use three gyros to cover any conditions of well deviation, with each gyro corresponding to an orthogonal axis X, Y, or Z. In operation, when using a gyro having a performance grade such as with a MEMS gyro, each gyro needs to be flipped by 180 degrees and measures in two directions for each axis to cancel out the sensor bias offset. Thus, in total, the gyros need to measure the angular velocity for 6 directions.
In the case of downhole-deployed geophones, a gimbal mount is often used to maintain the geophones in the preferred positions like vertical or horizontal, independent from the well deviation and the tool posture. A conventional passive gimbal mount system depends on gravity force to maintain the position of the spherical geophone mount which is floating in oil. Sometimes, devices such as omni-tilt geophones are used instead of the gimbal mounted geophone, because the conventional passive gimbal is expensive and has mechanical reliability issues as it's fragile and it's easy to be stuck especially at high temperature. However, some users still believe that the use of omni-tilt geophones tends to result in less confidence in the seismic signal vector fidelity than when using gimbal mounted geophones.
As will become apparent from the following description and discussion, the present invention overcomes at least some of these deficiencies and provides an improved pressure transducer housing.